I. Field of the Invention
The present invention generally pertains to mixing drums for batch style tilt-mixers particularly those used to mix erosive materials such as concrete prior to its being loaded into transit carriers including concrete mixer trucks, commonly known as readymix trucks. More specifically, the present invention relates to improvements in the drum drive and support design which enhance drum operation.
II. Discussion of Related Art
Tilt mixers having drums of various types have been used for many years in the construction industry for mixing batches of concrete to be loaded into trucks which, in turn, transport the concrete to job sites for placing. Over the years, experience has shown it to be most economical to manufacture tilt mixers themselves in a size capable of being transported by truck over the highways and mounted for use at the mixing plant. Therefore, they must be within the maximum size which can be legally transported on a highway. Tilt mixers are typically installed in an elevated position so that trucks hauling the pre-mixed materials may be loaded from above by tilting the mixer and discharging the mixed batch through a chute into the charging end of the truck.
Such devices typically are designed to be charged and to conduct the mixing operation in a substantially horizontal position. The charging end of the drum closely addresses a fixed loading chute for receiving measured amounts of aggregate, cement, sand and water, in the case of concrete, according to the batch formula used. The charging end of the drum must be configured so as to clear the chute when the drum is tilted to discharge the mixed materials. The, clearance with respect to the front or discharge end of the mixing drum is also important. Consideration of the height necessary to mount the drum for tilt loading of ready mix trucks and the clearance for the charging chute are important considerations which must be weighed together with the desire to make the drum itself as close as is reasonable to the ideal mixing shape, in which the length is the same as or approaches the diameter of the drum.
Most previous mixing drums of the class described have had to provide a cone with a severe angle to accommodate the charging chute and have generally been shaped with a reduced diameter with respect to length. They have also had to use a pivot point which is removed some distance from the discharge end of the drum with respect to the axis about which the drum is tilted in order to reduce the torque required to tilt the drum for discharging materials. This had led, to the development of a general drum configuration for tilt mixer drums which has become somewhat of an industry standard. It includes ends having rather radical conical sections and a rather elongated central mixing zone. The drum is mounted to be pivoted at a distance from the discharge end. The drum itself has to be mounted at a relatively high position in order for the truck loading-unloading chute beneath the discharge end of the mixer to clear the tops of the trucks which drive under the mixer for loading as the loading system must accommodate the mixing drum when fully tilted to discharge the mixed material.
In addition to the need to decrease the length to diameter ratio of the mixing drum itself, and to move the tilt axis forward to increase clearance, other aspects of the drum support and drive systems of prior mixers have certain drawbacks which have been sources of problems. First, prior drum central support rollers have consisted of a pair of single rollers riding in a track circumscribed about the drum. These two points of contact are required to support the great majority of the weight of the drum which, when loaded, may approach twenty (20) tons. These rollers frequently fail and must be replaced which causes the loss of valuable mixing time. Second, the drums, which are designed to rotate in one direction, are traditionally driven by a large ring gear having teeth adapted to be addressed by drive gears attached to the shafts of a pair of drive motors flanking the front of the mixer. These drives are typically very noisy. There must be sufficient play in the gears to accommodate slight misalignments, and starting and stopping. In present systems, this causes noise and tends to cause undue wear on the gears; and a better way to address the drive problem has been sought.